pcp2influxdb: clarify diagnostics for certain failure conditions
pcp2influxdb: clarify diagnostics for certain failure conditions

Add some more helpful error messages.  It now prints warnings in the
following situations that it did not before:

* The python requests package is not installed (also hard exits after
      printing the message)
* The HTTP request returns 404 (most likely the requested database does
      not exist)
* The connection fails

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp